[feature/frontend] Let admins send test email to validate SMTP config (#2934)

* [feature/frontend] Let admins send test email to validate SMTP config

* wee

docs/admin/settings.md

@@ -66,6 +66,10 @@ Instance administration settings.
 
 Run one-off administrative actions.
 
+#### Email
+
+You can use this section to send a test email to the given email address, with an optional test message.
+
 #### Media
 
 You can use this section run a media action to clean up the remote media cache using the specified number of days. Media older than the given number of days will be removed from storage (s3 or local). Media removed in this way will be refetched again later if the media is required again. This action is functionally identical to the media cleanup that runs automatically.

docs/api/swagger.yaml

@@ -4894,6 +4894,11 @@ paths:
                 - description: The email address that the test email should be sent to.
                   in: formData
                   name: email
+                  required: true
+                  type: string
+                - description: Optional message to include in the email.
+                  in: formData
+                  name: message
                   type: string
             produces:
                 - application/json

docs/configuration/smtp.md

@@ -6,6 +6,8 @@ Configuring GoToSocial to send emails is **not required** in order to have a pro
 
 In order to make GoToSocial email sending work, you need an smtp-compatible mail service running somewhere, either as a server on the same machine that GoToSocial is running on, or via an external service like [Mailgun](https://mailgun.com). It may also be possible to use a free personal email address for sending emails, if your email provider supports smtp (check with them--most do), but you might run into trouble sending lots of emails.
 
+To validate your configuration, you can use the "Administration -> Actions -> Email" section of the settings panel to send a test email.
+
 ## Settings
 
 The configuration options for smtp are as follows: